How to fix Error: cluster.fork() can only be called from the master process in Node.js

const cluster = require('cluster');
const os = require('os');

// CORRECT: Fork only in primary process
if (cluster.isPrimary) {
  console.log(`Primary process ${process.pid} is running`);

  // Fork workers equal to CPU cores
  const numCPUs = os.cpus().length;
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died`);
  });
} else {
  // Worker process - NO cluster.fork() here
  console.log(`Worker ${process.pid} started`);
  // Your application logic goes here
}

// Old (deprecated but still works)
if (cluster.isMaster) {
  cluster.fork();
}

// New (recommended)
if (cluster.isPrimary) {
  cluster.fork();
}

const cluster = require('cluster');
const express = require('express');
const os = require('os');

if (cluster.isPrimary) {
  // PRIMARY PROCESS CODE ONLY
  console.log(`Primary ${process.pid} is running`);

  // Fork workers
  for (let i = 0; i < os.cpus().length; i++) {
    cluster.fork();
  }

  // Handle worker events
  cluster.on('exit', (worker, code, signal) => {
    console.log(`Worker ${worker.process.pid} died, spawning new worker`);
    cluster.fork(); // OK to fork here - still in primary
  });

} else {
  // WORKER PROCESS CODE ONLY
  // NO cluster.fork() calls allowed here
  const app = express();

  app.get('/', (req, res) => {
    res.send(`Worker ${process.pid} handled request`);
  });

  app.listen(3000, () => {
    console.log(`Worker ${process.pid} listening on port 3000`);
  });
}

// shared-module.js - WRONG
const cluster = require('cluster');

function init() {
  cluster.fork(); // ERROR: Will fail if called from worker
}

// shared-module.js - CORRECT
const cluster = require('cluster');

function init() {
  if (cluster.isPrimary) {
    cluster.fork(); // Only forks if called from primary
  }
}

const cluster = require('cluster');

console.log('Is Primary:', cluster.isPrimary);
console.log('Is Worker:', cluster.isWorker);
console.log('Worker ID:', cluster.worker?.id || 'N/A');
console.log('NODE_WORKER_ID:', process.env.NODE_WORKER_ID);

// cluster.isPrimary is true when NODE_WORKER_ID is undefined
// cluster.isWorker is true when NODE_WORKER_ID is defined